Mole hammer-cycle control

ABSTRACT

This disclosure describes a scheme for switching the propelling fluid forces alternately to opposite sides of a Mole impacting hammer. The scheme is grounded on valving structures for sensing fluid pressure differences (1) between supply and return pressure and (2) on opposite sides of the hammer; and on orifice means for generating a controlled difference between (1) and (2). By selecting operating parameters in this system, the fluid force switching is always actuated at a point in the retract portion of the cycle so as to assure maximum hammer stroke.

United States Patent 11 1 Coyne Sept. 18, 1973 4] MOLE HAMMER-CYCLECONTROL 3,642,076 7/1970 Coyne et al 173/91 [75] inventor: JamesChristopher Coyne, New

providence, Primary ExaminerErnest R. Purser Attorney-W. L. Keefauver[73] Assignee: Bell Laboratories Incorporated,

Murray Hill, NJ. [57] ABSTRACT [22] Filed: Dec. 30, 1971 This disclosuredescribes a scheme for switching the [21] PP 214,342 propelling fluidforces alternately to opposite sides of a Mole impacting hammer. Thescheme is grounded on 52 U.S. c1 173/91, 91 /282, 173/134, valvingStructures for Sensing fluid pressure differences 75 (1) between supplyand return pressure and (2) on op- [51] lint. Cl E2lb 3/12 Posite Sidesof the hammer; and orifice means for [58] Field of Search "NB/13 51 319]; generating a controlled difference between (1) and 9 2 2 2 3 29 92;175/19 (2). By selecting operating parameters in this system, the fluidforce switching is always actuated at a point 5 References Cited in theretract portion of the cycle so as to assure maxi- UNITED STATES PATENTSmum hammer 1,352,469 9/1920 Nell 91/282 5 Claims, 6 Drawing FiguresFORWARD DRIVE RETRACT STROKE POWER SPOOL 4| PILOT 43 SPOOL 42 8 FRONTHAMMER Patented Sept. 18, 1973 4 Sheets-Sheet 2 Patented Sept. 18, 19733,759,335

F 4 Sheets-Sheet 3 RETRACTSTROKE POWER STROKE1 I FIG 3 SURGE T I M E'SUPPLY LINE OPERATE TIME Pm PRESSURE l8 TURNAROUND l6 PO'NT IMPACT EE LPH f l9 P l7 P I6 2% w A TIME FIG. 4

PI3=PI8 PI7 l2 l6 l9 P :P P A I 2 PILOT SPOOL 42 i l L l8 l9 PDU ACTSTROAE Ie H PIT P LLL, P18 L P19 VALVE FIG. 5 4I AND 42 OPERATE TIME+PL(A2A|) TE IE EE -RETI=IAcT STROKE Lu EC 0 8 TIME FORCE To LEFT P (AA) L 2 ONE CYCLE Patented Sept. 18, 1973 3,759,335

4 Sheets-Sheet 4 FIG. 6

TWO POSITION FOUR WAY REVERSING VALVE MOLE HAMMER-CYCLE CONTROL FIELD OFTHE INVENTION This invention relates in general to self-propelled earthpenetrators known as Moles; and more specifically to a Mole hammer cyclecontrol.

BACKGROUND OF THE INVENTION Self-propelled earth penetrators or Molestypically are driven by an internal linearly impacting hammer that isfree to slide for and aft. Between impacts of the hammer on a frontanvil, the hammer must return to the rearmost possible position withoutimpacting on the rear anvil, and without aid of bumpers or snubbers tostop the hammer motion. Accordingly, the hammer cycle must be completelycontrolled by the application and removal of hydraulic pressure at theproper times.

More specifically, after impact on the front anvil, at some propermidstroke point in the retract stroke while the hammer is beingaccelerated toward the rear anvil, the net force on the hammer must beswitched to cause a further impact on the front anvil. Thus, propulsionpressures are switched, the hammer decelerates to a stop, reverses itsdirection of motion, and then accelerates toward the front anvil.

For forward penetration into the soil, the rear anvil must of course notbe impacted; although it is desired that the maximum available strokedistance be utilized for hammer acceleration in the forward direction.

Thus the problem is one of switching the forces acting on the hammer atjust the right time to make the hammer coast to a stop just beforestriking the rear anvil. At end-of-stroke, after the hammer has impactedthe front anvil, the forces must again be switched to restore theinitial pressure conditions and commence the next cycle.

In practice, soil conditions, oil temperature, pump pressure, internalleakage rate and many other conditions will vary. Thus, if an extremelyclose control of hammer stroke over a wide range of operating conditionsis desired, a closed-loop feedback system using some sort of proximitycenters to generate an error signal to cause a correction to the pointof switch on the next cycle, would be necessary. However, need for aclosed-loop feedbacksystem could be obviated if a dependable open-loopsystem existed.

Accordingly, one object of the invention is to retain the maximum hammerstroke length over all conditions of soil, oil temperature, viscosity,hydraulic frictions, etc., but without striking the rear anvil.

A further inventive object is to maintain tunnel traction as the Molehammer cycles, so that the Mole does not slip back in the tunnel at anypoint.

A further inventive object is to achieve the foregoing objects with anopen-loop system.

A further inventive object is to obviate the need for position sensorsand mechanical linkages to the hammer.

A still further inventive object is to provide for hammer cycle reversalremotely, by interchanging at the ground surface supply and returnhydraulic hoses.

SUMMARY OF THE INVENTION It has been realized that a switching parameterG vlp exists that can be used as the basis for initiating the midstrokeswitch of net actuating (or hydraulic forces on the hammer, where v isthe instantaneous hammer velocity and p is the instantaneous value ofline pressure.

Although the inventive switching function would appear difficult toimplement at first glance, it is simple because of the proportionalitythat exists between turbulent flow pressure losses, and flow ratesquared.

The invention and its further objects, features, and advantages will befully apprehended from a reading of the description to follow of anillustrative embodiment.

BRIEF DESCRIPTION OF THE DRAWING FIGS. 1 and 2 are schematic hydrauliccircuit diagrams;

FIG. 3 is a graph tracing certain critical pressure variations;

FIG. 4 is a schematic diagram of the pilot spool which furtheridentifies certain critical pressures;

FIG. 5 is a graph depicting force on pilot spool versus time; and

FIG. 6 is a reversing valve.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENT Theory Equation 2 belowstates the proportionality between turbulent flow (through a turbulentflow resistor such as an orifice), pressure losses, and flow ratesquared.

PR Iq (2) where the pressure loss across the turbulent flow resistor, qflow rate, and C is a constant depending on the geometry of the orifice.The proportionality of flow rate q to hammer velocity v is:

where a is the pressurized area of the hammer. Substituting Equation (2)and (3) into Equation (1) gives:

G q pt PR/G m (4) where p, is line pressure. Equation (4) can be putinto slightly different form by making the substitution Pa P1. Pl: (5)

where p is the pressure difference across the hammer. This gives:

G P1. ph/ l PL (6) Accordingly, pursuant to the invention the hammerforces are switched when:

PL z t/C1 PL 2 k (7) where k is a specified constant. 1

Pursuant to one further step of algebraic simplifica? tion the hammerforces are switched when:

ph S PL (8) where C is a constant defined by:

C 1 C, a k 9 Implementation FIG. 1 is a complete schematic of thevalving system pursuant to the present invention for cycle control ofthe Mole. The power valve 1 and pilot valve 2 can each be in one of twopositions by virtue of the respective positions of power spool 41 andpilot spool 42. Thus,

taken together, the two valves 1 and 2 can be in four possible states.Each of these four states of the valves 1 and 2 corresponds to one ofthe four possible states of a linearly impacting hammer 3 as follows:

Hammer State Con- Pilot Power dition Spool Spool Drive Stroke I leftright forward retract 2 right left forward power 3 right right reverseretract 4 left left reverse power FIG. 1 shows the valves 1, 2 in thefirst condition listed above: forward-drive retract-stroke. Pressurizedoil enters the power valve 1 from a supply line 5 and leaves throughline 9 in which is located an orifice 13. The pressure drop acrossorifice 13 is proportional to q (the flow-rate squared). The flow passesthrough a line 10 to chamber 14 where the pressurized oil acts on anarea a of the hammer 3 determined by the diameter of chamber 14, thuspropelling the hammer toward rear anvil 32. Concurrently oil in chamberflows through line 11 to line 7 and through orifice 12 to the powervalve 1. The pressure drop across orifice 12 is also proportional to q(the flow-rate squared).

The flow leaves the power valve 1 through return line 4 which carriesthe oil flow back to a sump (not shown). The lines 4, 5, 7, 9, 10, 11carry the main propulsion flow and have large flow area (at leastonefourth inch I.D.) to minimize pressure losses. Lines 4 and 5 are longlengths of hose (250 feet or more), and are of one-half inch I.D.

In addition to these large power lines there are several small switchinglines going to the pilot valve 2. The pressure in chamber 14 iscommunicated via line 17 to the right-hand annular area denoted 43 ofthe pilot spool 2. Likewise the pressure in chamber 15 is communicatedvia line 16 to the left-hand annular area 44 of the pilot spool. Thepressure in line 7 is communicated via line 19 to the right-handstub-area 45 of the pilot spool 2. Likewise the pressure in line 9 iscommunicated via line 18 to the left-hand stub-area 46 of the pilotspool 2. The annular areas 43, 44 on either side of the pilot spool aresquare; also the stub areas 45, 46 are equal. Thus, Equations (10) and(l 1) may be written:

Area 43 Area 44 A (10) Area 45 Area 46 A 11 The ratio of Areas AJA is animportant parameter in the system. This ratio pursuant to one aspect ofthe invention, must be less than unity. Thus:

The pilot valve 2 has the function of switching high and low pressuresto either side of the power spool 41. Lines 21 and 22 connect supplypressure from line 5 to the inlet of pilot valve 2. Likewise, lineconnects return line pressure in or from line 4 to the inlet of thepilot valve 2. Lines 24 and 23 connect the outlet of the pilot valve 2to the right-hand end area denoted 47 and the left-hand end area denoted48 of the power spool 41.

In the forward-drive retract-stroke state reflected in FIG. 1, theleft-hand side of the power spool 41 is pressurized. Thus the powerspool 41 will be in the righthand position as shown in the Figure.

The changes in pressures on the pilot spool areas 43, 44, 45, 46 duringa retract stroke will now be considered. At the beginning of the retractstroke the hammer 3 velocity is zero. Hence there is no flow throughorifices 12 and 13 and the pressure in line 7 equals that in chamber 15.Likewise, the pressure in line 9 equals that in chamber 14. In otherwords referring to the FIG. 3 diagram, the pressure difference p acrossthe hammer 3 equals the net line pressure p,,. Hence, the force on thepilot spool 42 is equal to p (A A,) and is directed to the left. Thepilot spool 42 is'in the left-hand position as shown in FIG. 1.

The FIG. 3 diagram shows the changes of the four pressures acting on thefour areas of the pilot spool 2.

As the hammer 3 gains speed, the pressure losses across orifices 12 and13 increase. As shown on FIG. 3 the pressure difference p p (subscriptnumerals refer to oil lines) as well as the pressure difference (p pboth increase, due mostly to the throttling effect of the orifices 12and 13.

As is evident from FIG. 3, the pressure difference across the hammer 2decreases at a faster rate than the net line pressure p,,. Since p actson the larger areas of the pilot spool 42, a point will be reached wherethe force on the pilot spool 42 reverses direction. At this point thepilot spool will shift, thereby initiating the midstroke operation ofthe valves.

The forces acting on the pilot spool 42 will now be examined todetermine the critical point at which the pilot spool 42 operates. FIG.4 shows the pilot spool 42 and the four pressures acting on its areas.

The force tending to push pilot spool 42 to the right PL 1 PH 2 F (PisPls) i (P11 P1s) 2 This force is shown graphically versus time in FIG.5. The critical point at which the force becomes positive is found bysetting F (Equation 14) equal to zero. Thus, the pilot valve oprerateswhen:

This is the desired switching criterion previously discussed. It is seenthat the ratio A lA must be a positive fraction. Advantageously, it hasbeen found that this ratio is optimal if falling in the range of 0.6 to0.9.

As the pilot spool 42 shifts to the right, the pressure in line 24 riseswhile the pressure in line 23 drops, and the power spool 41 starts toshift to the left. High pressure oil entering the power valve from line5 now leaves the valve through line 8, by-passing the orifice 12. Theoil flows through line 11 to chamber 15, where it collides with theoncoming hammer 3, causing the pressure surge denoted p in FIG. 3. Atthe same time, line 10 is being connected through the power valve 1 toreturn line 4; and the pressure in line 10 and chamber 14 drops toreturn line pressure. The hammer, however, continues its motion to theleft causing oil to be drawn into chamber 14 from the return line 4.This causes the oil pressure in chamber 14 to drop further, to a valueless than return line pressure.

The power spool 41, having shifted to the left, blocks flow throughlines 7 and 9 in or out of the power valve 1. The flow by-passes theorifices 12, 13 and consequently the pressures in lines 7 and 9 becomeequal to the pressures in lines 11 and respectively. The pressures p andp differ now by only the amount of pressure loss in the valve 1 andlines. As FIG. 3 shows, p, differs only slightly from p after themidstroke valve operation. Likewise p differs only slightly from p Thesmall differences are due to pressure losses in the valve and lines.

A second result of the power spool shifting to the left is that ofinterchanging the pressures in lines 18 and 19. P which had beensupply-line pressure now becomes return-line pressure and p which hadbeen return-line pressure now becomes supply-line pressure. Thus, thenet line pressure is still applied on the stub areas of the pilot spoolbut now with opposite polarity. Since the polarity of both p, and 12,,across the pilot spool 42 has been reversed, the force tending to pushthe pilot spool to the right becomes the negative of Equation (13).Thus; -pt 1 p" 2 (16) The pilot spool becomes latched to the right as inFIG. 2 because A A and p p,,. The second inequality is true because thehammer 13 is being decelerated by pressure of the supply line 5. At thispoint in the cycle the hammer 3 is pumping into the supply line5 anddrawing oil out of the return line 4. Naturally, p will exceed the netline pressure. Also, the pressure surge mentioned earlier, occasioned bythe rapid opening of the supply line 5 into the oncoming hammer 3, helpsto latch the pilot valve to the right.

For reasons discussed above, theforce on the pilot spool 42 during themidstroke valve operation rises very rapidly from zero to a value inexcess of p (A -A as shown in FIG. 5, thereby latching the pilot spool42 to the right. As the hammer 3 decelerates, the latching forcediminishes. The hammer 3 coasts to a stop at a point just short of therear anvil 32, reverses direction, and begins to accelerate toward thefront anvil. At the hammer turn-around point, the latching force on thepilot spool equals p (A A As the hammer 3 accelerates forward, thelatching force decreases due to the pressure losses in the valve andlines. Since the orifices l2, 13 are by-passed, the latching force doesnot decrease as rapidly as it didduring the retract stroke.

The operation of the end-of-cycle valve operation will now be described.Just before the hammer 3 impacts the front anvil 31, seal 25 closes offthe opening from chamber 14 into line 17. Leakage of high pressure oilthrough line 27 and orifice 28 into line 17 elevates the pressure in.line 17, thereby increasing the force on the right-hand annular area 43of the pilot spool. The pressure in line 17 builds up at a rate governedby the orifice 28 size and the oil compressibility. At some point intime (for example after impact has occurred) the net force on the pilotspool reverses direction, shifting the pilot spool 42 to the left. Thepressure level in line 17 at which the pilot spool shifts to the left isapproximately equal to (l A,/A, times line pressure.

The shifting of the pilot spool 42 to the left interchanges high and lowpressure on the ends of the power spool 41, thereby causing the powerspool to shift to the right. This power valve operation pressurizes line10 and de-pressurizes line 11. The leakage flow through line 27 andorifice 28 reverses direction. Since the opening from chamber 14 intoline 17 is still closed, the pressure in line 17 would'leak away andallow the pilot spool to shift back to the right if it were not forcheck valve 29. The function of check valve 29 is that of providing apressure path from line 10 to line 17 thereby maintaining the netleftward directed force on the pilot spool immediately after theend-of-cycle valve operation. I

The re-pressurization of chamber 14 causes the hammer 3 to accelerateagain toward the rear anvil 32. The hammer seal 25 moves away, reopeningline 17 into chamber 14 and the system is back in its original statehaving gone through one complete cycle.

The spring of pilot valve 42 has two functions. The first is tocompensate for the small variations in stroke versus supply linepressure caused by actual (nonideal) conditions, primarily the finiteoperate time of the spools 41, 42. Experiments have shown that a hammerstroke variation of less than 10 percent can be achieved over a 2:1range in supply line pressure with use of the proper choice of springand precompression. The second function of spring 60 is to cause a shortstroke, and hence a weaker impact, when the Mole is in the reverse modeof operation. For backing out of a tunnel, a high energy impact is notnecessary; and in fact is undesirable because of wear and fatigue ofparts. Advantageously, the spring 60 causes the hammer to couple with ashort stroke at a rapid cycle rate in the reverse mode.

The detailed description given above is for the forward drive mode ofoperation. To operate the Mole in reverse it is only necessary tointerchange the'supply and return lines at the pump using, for example,a standard two-position four-way valve 60 shown in FIG. 6. Then thehammer will impact at the rear anvil 32 instead of the front anvil 31.In this reverse drive the operation of the valving system issubstantially identical to that for the forward drive mode. The basicsystem is symmetrical. Thus, for example, check valve 30 performs thesame function in the reverse drive'as check valve 29 does in the forwarddrive.

It is to be understood that the embodiments described herein are merelyillustrative of the principles of the invention. Various modificationsmay be made thereto by persons skilled in the art without departing fromthe spirit and scope of the invention.

What is claimed is:

1. In a Mole'comprising an internal front anvil, a hammer having frontand rear ends, pressurized fluid drive means having a power stroke modeand a retract stroke mode and including a fluid switch alternatelyapplying said fluid to said hammer rear and front ends to respectivelyaffect said two modes, and wherein said hammer travels between saidfront anvil and a retracted position defining therebetweenan optimumhammer stroke length, the improvement in controlling the operation ofsaid fluid switch comprising:

means for continuously deriving a switching parameter G vlp where v isthe instantaneous velocity of said hammer and p is the instantaneouspressure of said fluid, and

means responsive to sensing when saidparameter G reaches a predeterminedvalue, said fluid switch, said predetermined value being selected toassure said optimum hammer stroke length.

2. The apparatus of claim 1, further comprising a rear anvil rearwardlyadjacent to said retracted position, and wherein said fluid drive meansfurther comprises means for efiecting hammer impact on said rear anvilonly, by the same said controlled operation of said fluid switch.

3. A Mole system comprising:

an elongated body,

a front anvil linked to said body,

a hammer having from and rear hydraulic drive ends,

means for mounting said hammer within said body for linear movementbetween said front anvil and a retracted position defining therebetweena maximum hammer stroke length physically permissible,

pressurized hydraulic drive means having a power stroke state and aretract stroke state and comprising supply and return hydraulic lines,

means, operative only during said retract stroke and including anorifice connected in series with said hydraulic drive means, forgenerating a pressure difference P P where P is the difference betweensaid supply and said return line pressures, and P is the difference inpressure between said hammer front and rear ends,

means including power spool means for alternately supplying hydraulicfluid from said supply line to said hammer rear and front ends torespectively affect said two modes, and

pilot valve means having first and second states, and connected to saidpower spool means and having a first pair of opposed pressure regionseach of area A connected to said hammer front and rear ends,

a second pair of opposed pressure regions each of area A connected tosaid supply and to said return lines, where the ratio AJA is a positivefraction,

said pilot valve means further comprising means operating when (A /A P P0, to effect a switch in said states, thereby to switch said drivemeans, from said power stroke mode to said retract stroke mode, and

third means responsive to a predetermined value of difference betweensaid first sensed pressure and said second sensed pressure the latterscaled by the factor A /A sensed by said first and second sensing meansfor operating said pilot spool means.

4. Apparatus pursuant to claim 3, further comprising means includingmovement of said power spool means for placing said orifice means inseries with said supply line during said retract stroke, said orificemeans being by-passed by said supply line flow during said power stroke,means connecting said orifice means during said retract stroke to saidhammer front end, said orifice developing a pressure differencethereacross that is proportional to the velocity squared of said hammer,the proportionally constant being selected to assure said maximum hammerstroke length physically permissible and wherein the ratio A,/A is in arange between 0.4 and 0.7.

5. Apparatus pursuant to claim 3, further comprising a rearanvilre'arwardly adjacent to said retracted position, and wherein saidfluid drive means further comprises means for effecting hammer impact onsaid rear anvil only, by the same said controlled operation of saidfluid switch.

1. In a Mole comprising an internal front anvil, a hammer having front and rear ends, pressurized fluid drive means having a power stroke mode and a retract stroke mode and including a fluid switch alternately applying said fluid to said hammer rear and front ends to respectively affect said two modes, and wherein said hammer travels between said front anvil and a retracted position defining therebetween an optimum hammer stroke length, the improvement in controlling the operation of said fluid switch comprising: means for continuously deriving a switching parameter G v2/p where v is the instantaneous velocity of said hammer and p is the instantaneous pressure of said fluid, and means responsive to sensing when said parameter G reaches a predetermined value, said fluid switch, said predetermined value being selected to assure said optimum hammer stroke length.
 2. The apparatus of claim 1, further comprising a rear anvil rearwardly adjacent to said retracted position, and wherein said fluid drive means further comprises means for effecting hammer impact on said rear anvil only, by the same said controlled operation of said fluid switch.
 3. A Mole system comprising: an elongated body, a front anvil linked to said body, a hammer having front and rear hydraulic drive ends, means for mounting said hammer within said body for linear movement between said front anvil and a retracted position defining therebetween a maximum hammer stroke length physically permissible, pressurized hydraulic drive means having a power stroke state and a retract stroke state and comprising supply and return hydraulic lines, means, operative only during said retract stroke and including an orifice connected in series with said hydraulic drive means, for generating a pressure difference PL - PH, where PL is the difference between said supply and said return line pressures, and PH is the difference in pressure between said hammer front and rear ends, means including power spool means for alternately supplying hydraulic fluid from said supply line to said hammer rear and front ends to respectively affect said two modes, and pilot valve means having first and second states, and connected to said power spool means and having a first pair of opposed pressure regions each of area A2 connected to said hammer front and rear ends, a second pair of opposed pressure regions each of area A1 connected to said supply and to said return lines, where the ratio A1/A2 is a positive fraction, said pilot valve means further comprising means operating when (A1/A2) PL - PH > or = 0, to effect a switch in said states, thereby to switch said drive means, from said power stroke mode to said retract stroke mode, and third means responsive to a predetermined value of difference between said first sensed pressure and said second sensed pressure the latter scaled by the factor A1/A2 sensed by said first and second sensing means for operating said pilot spool means.
 4. Apparatus pursuant to claim 3, further comprising means including movement of said power spool means for placing said orifice means in series with said supply line during said retract stroke, said orifice means being by-passed by said supply line flow during said power stroke, means connecting said orifice means during said retract stroke to said hammer front end, said orifice developing a pressure difference thereacross that is proportional to the velocity squared of said hammer, the proportionally Constant being selected to assure said maximum hammer stroke length physically permissible and wherein the ratio A1/A2 is in a range between 0.4 and 0.7.
 5. Apparatus pursuant to claim 3, further comprising a rear anvil rearwardly adjacent to said retracted position, and wherein said fluid drive means further comprises means for effecting hammer impact on said rear anvil only, by the same said controlled operation of said fluid switch. 